Vaisala RVP900, User Manual

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The computation of
Total Power
is performed using the same subset of central IF
samples that are used to compute
Filtered Power
. This smaller subset of IF samples
comes about because filtering the data requires a convolution with the current FIR filter, and
this computation does not produce results all the way to the edges of the input data. This is
the same reason that the LOG plots do not extend across the full screen.
Because of this definition, it is valid to intercompare the
Total Power and Filtered
Power . The two numbers match exactly as long as all of the incoming power falls within the
passband of the FIR filter. The difference between the two powers can be used as a measure
of the filter loss for a given pulse shape, that is, the portion of signal that is lost outside of
the filter's passband.
The Total , Filtered , and MidSamp values represent true RMS power (that is,
variance), and not merely a sum-of-squares. Any DC offset present in the A/D converter
does not affect these power levels.
6.8
Pa
— Plot Tx Waveform Ambiguity
RVP900 can make radar observations using compressed pulse waveforms. This opens many
new opportunities for using low-power solid-state transmitters that employ very long pulse
lengths (20 ... 80 μsec). Transmitters of this kind are less expensive, both to build and to
maintain, compared with traditional magnetron or klystron systems.
However, the signal processing and waveform design required to make good use of these
long transmit pulses is also much more complex. To help with this, RVP900 provides the
Pa
(plot ambiguity) command in which compressed transmit waveforms can be designed,
studied, and optimized. In the
Pa , plots you can experiment with different waveform
designs, try out bandwidth and pulse width options, and examine and optimize the range/
time sidelobes of your waveform.
6.8.1 Interpreting Ambiguity Plots
The following figure shows one form of Pa plot in which the magnitude of the Tx/Rx range
sidelobes are drawn on a log scale having 10dB vertical ticks. The horizontal span of the plot
is equal to the length of the pulse, and consequently, only half of the complete ambiguity
diagram is shown. This was done to make the plots more viewable; and no information lost
since the zero-Doppler response (white plot) can safely be assumed to be symmetric. In this
example the pulse width is 30 μsec, bandwidth is 3 MHz, PSL is -61.2 dB and ISL is -50.8 dB,
Doppler shift ± 50 KHz.
RVP900 User Guide M211322EN-J
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